1. Field of the Invention
The present invention relates to an arc tube used as a light source etc. of a discharge lamp and a manufacturing method thereof.
2. Description of the Related Art
A discharge lamp is able to irradiate light with high luminance, so that the discharge lamps have been used in many cases as head lights for vehicles, illuminating lights for stores or the like as well as field illuminating lights and road illuminating lights. Arc tube shown in FIG. 1 is knows as a light source of such a discharge lamp.
The arc tube 2 is configured by a quartz glass tube 4 having a spherical portion 4a formed at the center portion thereof and a pair of electrode assemblies 6 provided at both sides of the spherical portion 4a within the quartz glass tube 4. Each of the electrode assemblies 6 is formed in a manner that an electrode rod 8 protruding within the inner space (discharge room) of the spherical portion 4a is coupled to a lead wire 10 protruding from the end portion of the quartz glass tube 4 through a rectangular molybdenum foil 12. Further, each of the electrode assemblies 6 is pinch-sealed by the quartz glass tube 4 at the molybdenum foil 12 portion.
The expression xe2x80x9cpinch-sealxe2x80x9d in the present specification means such a sealing method that a heated quartz glass tube is squeezed thereby to bury within the quartz glass tube insertion material (molybdenum foil etc.) placed within the quartz glass tube in a state that the insertion material is adhered to the material of the quartz glass tube.
Although the pair of the molybdenum foils 12 are sequentially pinch-sealed one by one, conventionally the pinch-seal process for the first one of the molybdenum foils has been performed in the following manner.
That is, as shown in FIG. 7, the electrode assembly 6 is inserted from the one end portion of the quartz glass tube 4 thereby to place the molybdenum foil 12 in the vicinity of the spherical portion 4a within the quartz glass tube 4 ((a) of FIG. 7). In this state, inactive gas such as argon gas, nitrogen gas or the like is flown into the quartz glass tube 4 thereby to exhaust the atmosphere within the quartz glass tube 4, and simultaneously a portion of the quartz glass tube 4 surrounding the molybdenum foil 12 is heated by a burner 20 ((b) of FIG. 7). Then, the quartz glass tube 4 is squeezed by a pincher 22 ((c) of FIG. 7) thereby to perform the pinch-sealing. As a result, the intermediate product of an arc tube shown in (d) of FIG. 7 can be obtained.
The aforesaid conventional pinch-seal method is arranged in a manner that, in order to prevent the reduction of tensile strength of the molybdenum foil (breakage of the foil) due to the oxidation of the molybdenum foil, the inactive gas is flowed into the quartz glass tube 4 thereby to exhaust the air causing the oxidation. However, the inner pressure within the quartz glass tube 4 at the time of the pinch-sealing is kept almost at the atmospheric pressure. Thus, as shown in FIG. 8, interfaces 12a between the pinch-sealed molybdenum foil 12 and the quartz glass tube 4 are kept in a planer shape which is relatively smooth and similar to the surface shape of the molybdenum foil 12 before the pinch-sealing.
However, the thermal expansion coefficient largely differs between the molybdenum foil 12 and the quartz glass tube 4. Thus, in the case where the interface 12a is a smooth planer shape, if the arc tube 2 is lightened or turned on, the exfoliation may likely occur between the molybdenum foil 12 and the quartz glass tube 4 due to the shearing stress xcfx84 caused by the difference of the thermal expansion coefficients therebetween. If such an exfoliation occurs, there arises a problem that the leakage may occurs at the arc tube 2 and hence the life time of the arc tube becomes quite short.
The present invention has been performed in view of the aforesaid circumstance and an object of the present invention is to provide an arc tube and fabricating method thereof which can effectively prevent the occurrence of exfoliation between a molybdenum foil and a quartz glass tube which causes leakage.
In order to attain the aforesaid object, in the present invention, the pinch-sealing for the first one of the pair of molybdenum foils is not performed by flowing inactive gas within a quartz glass tube like the prior art but performed in a manner that the one end portion of the quartz glass tube is sealed, then the pinch-sealing is performed while air within the quartz glass tube is exhausted from the other end portion thereof so that pressure within the quartz glass tube becomes in a negative pressure state of a predetermined pressure and a pinch seal estimation portion of the quartz glass tube is heated, whereby fine concave and convex portions are formed on the interfaces between the molybdenum foils and the quartz glass tube thus having been pinch-sealed thereby to place the molybdenum foils and the quartz glass tube in an engage state to each other.
That is, in this invention, the arc tube according to the present invention is characterized in that, in the arc tube wherein a pair of molybdenum foils are pinch-sealed at both ends of a spherical portion of a quartz glass tube, the surface roughness of the molybdenum foils at the interfaces between the molybdenum foils and the quartz glass tube is set to be 1 xcexcm (reference length of 0.08 mm) or more at ten-point average roughness.
In order to obtain such an arc tube, in this invention, the arc tube fabricating method according to the present invention is characterized in that, in a method for fabricating the arc tube, in which a pair of molybdenum foils are pinch-sealed at both ends of a spherical portion of a quartz glass tube, by sequentially pinch-sealing the pair of molybdenum foils, first one of the pair of molybdenum foils is pinch-sealed in a manner that the first one of the pair of molybdenum foils is inserted into the quartz glass tube thereby to seal one end portion of the quartz glass tube, and then a pinch seal estimation portion of the quartz glass tube is squeezed by a pincher while air within the quartz glass tube is exhausted from the other end thereof so that pressure within the quartz glass tube is in a negative pressure state of 100 torr or less and while heating the pinch seal estimation portion.
In this invention, xe2x80x9cthe interfaces between the molybdenum foils and the quartz glass tubexe2x80x9d means both main and rear surfaces of each of the molybdenum foils and the surface roughness of the end portions of each of the molybdenum foils is not particularly limited.
In this invention, the concrete method for xe2x80x9csealing the one end portion of the quartz glass tubexe2x80x9d is not particularly limited. For example, a method for heating and squeezing the one end portion of the quartz glass tube and heat-sealing by shrink-seal or the like, a method for choking the one end portion of the quartz glass tube by other member, or the like may be employed.
Since the arc tube according to the present invention is arranged in a manner that the surface roughness of the molybdenum foils at the interfaces between the molybdenum foils and the quartz glass tube is set to be 1 xcexcm (reference length of 0.08 mm) or more at ten-point average roughness, the coupling intensity between the quartz glass tube and the respective molybdenum foils can be made sufficiently high. Thus, since the molybdenum foils and the quartz glass tube engage to each other by means of the fine concave and convex portions formed on the interfaces and the contact area between the molybdenum foils and the quartz glass tube is increased, at the time of turning-on of the arc tube, the exfoliation between the molybdenum foils and the quartz glass tube due to the difference of the thermal expansion coefficients therebetween can be prevented from being occurred in. Thus, the leakage of the arc tube can be prevented and the life time thereof can be made longer.
In the arc tube fabricating method according to the present invention, first one of the pair of molybdenum foils is pinch-sealed in a manner that the first one of the pair of molybdenum foils is inserted into the quartz glass tube thereby to seal one end portion of the quartz glass tube, and then a pinch seal estimation portion of the quartz glass tube is squeezed by a pincher while air within the quartz glass tube is exhausted from the other end thereof so that pressure within the quartz glass tube is set in a negative pressure state of 100 torr or less and while heating the pinch seal estimation portion. Thus, the arc tube fabricating method according to the present invention has the following function and effects.
Since the pressure within the quartz glass tube is maintained in a negative pressure state of 100 torr or less, the inner wall surface of the pinch seal estimation portion thus heated is attracted to the molybdenum foil side. Thus, when the squeeze is performed in this state, the fine concave and convex portions are formed on the interfaces between the molybdenum foils and the quartz glass tube. Further, since the air within the quartz glass tube is continuously exhausted from the other end thereof until the completion of the heating and squeezing processes, unnecessary gas generated at the heating process from the inner portion of the material of the quartz glass tube and the electrode assemblies can be efficiently removed.
If the surface roughness of the respective molybdenum foils at the interfaces between the molybdenum foils and the quartz glass tube is set to be 1 xcexcm (reference length of 0.08 mm) or more at ten-point average roughness, the surface roughness of the molybdenum foils before the pinch-sealing is not limited particularly. However, when the surface roughness itself of the molybdenum foils is set to be 1 xcexcm (reference length of 0.08 mm) or more at ten-point average roughness, in addition to the function of forming the fine concave and convex portions due to the aforesaid fabricating method, fine concave and convex portions can be formed more surely on the interfaces between the respective molybdenum foils and the quartz glass tube in the completed product state of the arc tube.
The heated temperature of the pinch seal estimation portion is not limited to a particular temperature so long as the quartz glass tube can be melted. However, since the quartz glass starts melting about 1,700xc2x0 C., when the heated temperature is set in a range of 2,000xc2x0 C. to 2,300xc2x0 C., the inner wall surface of the pinch seal estimation portion can be effectively attracted to the molybdenum foil side by the negative pressure. As a consequence, the fine concave and convex portions can be formed surely on the interfaces between the respective molybdenum foils and the quartz glass tube.
As described above, in the fabricating method of the arc tube, the concrete method for sealing the one end portion of the quartz glass tube is not limited to a particular method. However, if the sealing process is performed by the provisional pinch-sealing process in which the one end portion of the quartz glass tube is sealed while sandwiching the part of the molybdenum foil, the positioning of the molybdenum foil at the time of the pinch-sealing performed thereafter can be performed surely and accurately.